Many cable television (CATV) systems generally transmit radio frequency electrical signals over a hybrid network with optical fiber communication channels interfaced with coaxial electrical cables. A plurality of user signals is simultaneously transported over long ranges over different portions of an RF spectral band. Each user signal can include analog video signals, broadcast digital video, video on demand and digital signals, which enable access through an internet gateway. To aggregate and format the plurality of signals for transmission over the (HFC) network, at least some of the signals are modulated into RF subcarriers using direct digital synthesis, and the RF signal is subsequently used to drive an optical transmitter to form the optical signal that is then transported along a fiber network.
A modulator converts a baseband digital signal input into a radio frequency signal. The radio frequency signal is used to modulate a laser to transmit the optical signal along the optical fiber network. The laser may be either directly- or externally-modulated. The laser is part of a component of the system that can be referred to as the optical transmitter. The modulators for converting the input baseband digital signals into a plurality of RF subcarrier signals each with QAM (quadrature amplitude modulation) for driving the optical transmitter are referred to in the art as edge-QAM devices. If the optical transmitter is appropriately packaged, the operator can replace the unit conveniently if the optical transmitter is damaged or if there is another desired change. While there is continuing demand for greater bandwidth available to individual users, there is also corresponding pressure to keep a low cost structure.